1. Field of the Invention
The present invention relates to a process for the preparation of a discoloration-free, high-molecular weight (co)polycarbonate which comprises melt-polycondensing a dihydroxy compound with a carbonic diester containing specific impurities in reduced amounts through transesterification. Particularly, the present invention relates to a process for the preparation of a discoloration-free, high-molecular weight (co)polycarbonate which comprises melt-polycondensing a dihydroxy compound with a carbonic diester containing specific impurities in reduced amounts through transesterification in the presence of (a) a nitrogen-containing basic compound and/or (b) an alkali metal compound and/or an alkaline earth metal compound as a catalyst(s).
2. Description of the Related Art Although a discoloration-free, high-molecular weight polycarbonate is a general-purpose engineering thermoplastic which is useful in various fields, particularly as injection molding material or sheet material substituting for a window pane, the processes for the preparation thereof according to the prior art has various problems.
The interfacial polycondensation process is generally effective in preparing a polycarbonate, but has disadvantages that the use of toxic phosgene is necessitated and that the formed polycarbonate is contaminated with residual chlorine ion.
In order to overcome these disadvantages, Japanese Patent Publication-A No. 63-182336 (published on Jul. 27, 1988) discloses a process for the preparation of a polycarbonate which comprises using liquid trichloromethyl chloroformate, which is a dimer of phosgene, instead of the toxic phosgene and polycondensing it with a special dihydric phenol by the interfacial process.
However, this patent document does not give any specific information about the special dihydric phenol with the exception of 9,9-bis(4-hydroxyphenyl)fluorenes. Further, although Angew. Chem. 99, p.p. 922-923 (1987) (by H. Eckert and B. Forster; "Triphosgen ein Kristalliner Phosgen-Ersatz") describes that a polycarbonate is prepared from 2,2-bis(4-hydroxyphenyl)propane by using triphosgene instead of the toxic phosgene, a reaction mechanism wherein phosgene is generated is also described therein.
Further, it is proposed in Japanese Patent Publication-A No. 4-100824 (published on Apr. 4, 1992) to use a diaryl carbonate having a xanthone content of 10 ppm or below. Although the use of such a carbonate can give a relatively light-colored polycarbonate, it has a problem that the obtained polycarbonate somewhat yellows as compared with that prepared by the phosgene process.
Furthermore, a process for the preparation of a polycarbonate using a catalyst system comprising a combination of a nitrogen-containing basic compound with a boron compound is proposed, for example, in U.S. Pat. No. 4,590,257 (date of patent: May 20, 1986, assignee: General Electric Co.). Although this process can give a relatively light-colored polycarbonate in spite of the poor activity of the catalyst on the polymerization, the process has problems that it takes a long time to complete the polymerization because of the low activity of the catalyst on the polymerization which results in low industrial productivity and not only a side reaction liable to occur during the polymerization to form a branched structure, but also the obtained polycarbonate is poor in heat stability. In addition to the U.S. Pat. No. 4,590,257 described above, U.S. Pat. Nos. 3,442,854 (assignee: FARBENFAB BAYER AG) and 5,276,109 (assignee: GENERAL ELECTRIC CO.; date of patent: Jan. 4, 1994), and European Patent Publication-A Nos. 351168 (published on Jan. 17, 1990) and 435124 (published on Jul. 3, 1991) disclose that a boron compound such as boric acid and triphenylborate is useful to prevent the discoloration of the polycarbonate.